Recently, development of radio base stations which can be placed indoors has been advanced. A cell formed by such a radio base station is generally called a femto cell since its cover area is extremely narrower than that of a radio base station placed outdoors. In the subsequent description, the radio base station forming the femto cell is referred to as a femto base station. By contrast, a radio base station forming a cell (generally called a macro cell) larger than the femto cell is referred to as a macro base station in some cases for differentiation from the femto base station.
Hereinafter, a structure and an operation of a typical mobile communication system where the femto base station is applied and communication is conducted in accordance with W-CDMA (Wideband Code Division Multiple Access) method will be described with reference to FIGS. 15 and 16.
As shown in FIG. 15, a mobile communication system 1z includes three units of femto base stations 10z_1 to 10z_3 which are placed adjacent to each other, and a mobile station 20z which selectively and wirelessly connects to these femto base stations 10z_1 to 10z_3.
The femto base stations 10z_1 to 10z_3 are connected by LAN (Local Area Network) or the like so as to be able to mutually communicate. Further, each of the femto base stations 10z_1 to 10z_3 operates a femto cell in accordance with a configuration from a maintenance device 2z. Note that the maintenance device is a typical user terminal such as a PC (Personal Computer).
In the operation, as shown in FIG. 16, the maintenance device 2z firstly allocates different PSCs (Primary Scrambling Codes) to the respective femto cells in accordance with user operations, and makes a cell configuration list 200 which indicates the status of allocation of the PSCs (Step S101). As shown in FIG. 15, the allocation of PSC=“C1” to the femto base station 10z_1, the allocation of PSC=“C2” to the femto base station 10z_2 and the allocation of PSC=“C3” to the femto base station 10z_3 are registered on the cell configuration list 200. In this case, the maintenance device 2z instructs the femto base station 10z_1 to use the PSC=“C1” (Step S102_1), instructs the femto base station 10z_2 to use the PSC=“C2” (Step S102_2), and instructs the femto base station 10z_3 to use the PSC=“C3” (Step S102_3).
Next, the maintenance device 2z extracts neighboring femto cells of the respective femto base stations 10z_1 to 10z_3 from the cell configuration list 200, and makes neighboring cell configuration lists 201_1 to 201_3 which indicate the status of allocation of the PSCs in the extracted neighboring femto cells (Step S103). As shown in FIG. 15, the allocation of PSCs=“C2” and “C3” respectively to the femto base stations 10z_2 and 10z_3 is registered on the neighboring cell configuration list 201_1. Further, although illustration is omitted, the allocation of PSCs=“C1” and “C3” respectively to the femto base stations 10z_1 and 10z_3 is registered on the neighboring cell configuration list 201_2, and the allocation of PSCs=“C1” and “C2” respectively to the femto base stations 10z_1 and 10z_2 is registered on the neighboring cell configuration list 201_3. Then, the maintenance device 2z notifies the femto base station 10z_1 of the neighboring cell configuration list 201_1 (Step S104_1), notifies the femto base station 10z_2 of the neighboring cell configuration list 201_2 (Step S104_2), and notifies the femto base station 10z_3 of the neighboring cell configuration list 201_3 (Step S104_3).
On the other hand, the femto base stations 10z_1 to 10z_3 respectively use the PSCs=“C1” to “C3” instructed by the maintenance device 2z to wirelessly transmit broadcast information.
Assume that the mobile station 20z is located within the femto cell formed by the femto base station 10z_1. In this case, the mobile station 20z receives the broadcast information transmitted from the femto base station 10z_1, thereby wirelessly connecting to the femto base station 10z_1 (Step S105). Thus, the femto base station 10z_1 and the mobile station 20z are in communication with each other (Step S106).
After that, when the mobile station 20z moves to approach the femto base station 10z2 as shown in FIG. 15, the mobile station 20z detects PSC of a neighboring femto cell (hereinafter, referred to as neighboring cell PSC) 203=“C2” as a result of neighboring cell search (Step S107). At this time, the mobile station 20z notifies the neighboring cell PSC 203=“C2” to the serving femto base station 10z_1 (Step S108).
The femto base station 10z_1 refers to the neighboring cell configuration list 201_1, thereby specifying the femto base station 10z_2 to which the same PSC=“C2” as the neighboring cell PSC 203 is allocated as a handover target of the mobile station 20z (Step S109). At this time, the femto base station 10z_1 requests the femto base station 10z_2 to accept a handover of the mobile station 20z (Step S110). Further, the femto base station 10z_1 requests the mobile station 20z to perform the handover to the femto base station 10z_2 (Step S111). Thus, the femto base station 10z_2 and the mobile station 20z are in communication with each other (Step S112).
Note that NPL 1, for example, discloses the above-mentioned method of specifying the handover target cell (base station) based on the PSC.